Capacitive detectors for detecting an object concealed in a wall, such as a beam concealed in a wall, have been disclosed in the prior art. These frequently use an electrode, the charging behavior of which is observed in order to home in on the dielectric object. Detectors with a plurality of electrodes, wherein a change in the capacitance of electrode pairs is determined, have also been disclosed. It is usually necessary to calibrate these detectors manually on the wall, as the units themselves are not able to detect contact with the wall, and the capacitance depends on ambient conditions such as a dampness of the wall, a humidity or an ambient temperature. It is therefore not possible to calibrate detectors of this kind once during manufacture; instead the calibration must be carried out by a user relatively shortly before a measurement. In doing so, the result of a measurement depends on the quality of the calibration. If the detector is calibrated with wall contact at a point where a beam is located, a later measurement result can be ambiguous, which can confuse the user.
Furthermore, the detectors described are prone to tilting on the wall. As a result of tilting, the distances of a plurality of electrodes from the wall are unequal, so that measured capacitances are subject to error. In addition, the capacitance measured at the electrodes when the detectors described approach the wall does not increase steadily, so that the distance from the wall and an approach or distancing of the detector from the wall can be difficult to reproduce.
DE 10 2008 005 783 A1 describes a moisture-independent capacitive device for protecting against trapping. The trapping sensor comprises two electrodes with variable spacing. The trapping sensor and a circuit with constant capacitance are supplied with alternating voltages, wherein the voltages are amplified or attenuated so that a voltage at one of the electrodes of the trapping sensor is minimized. If the relative spacing of the electrodes is changed, then trapping is detected based on an increased voltage at the electrode.
The disclosure is based on the object of providing a simple and accurate detector for a dielectric object, e.g. a wooden beam in a wall.